In recent years, from the perspective of public health and food safety, there has been an increasing need for inspection of foreign matter that may be contained inside food products.
While the methods of X-ray inspection are numerous, an inspection method that is receiving attention is a method in which X-rays are used to collect information on a substance inside a food product. As an example for achieving the foregoing, a so-called in-line-type X-ray inspection apparatus is known. In the in-line-type X-ray inspection apparatus, an X-ray tube and a detector are arranged above and below a conveyor belt that is sandwiched therebetween. The in-line-type X-ray inspection apparatus uses X-rays to inspect a food product to be inspected that is placed on the belt. In the case of this apparatus, the food product to be inspected is placed on the belt (line) and conveyed such as to pass through an X-ray radiation field of the X-ray tube. The X-rays that are transmitted through the passing food product are detected by the detector on the underside of the belt. An image is then generated based on the detection data. As a result of image processing being performed on the generated image by software, the presence and the type of foreign matter that may have become mixed into the food product can be discovered. In addition, the target of inspection is not limited to foreign matter. The inspection may target an object in which a difference in contrast occurs through X-rays, and of which the size, shape, or weight is required to be more accurately determined.
Therefore, this in-line-type X-ray inspection apparatus is suitable for instances in which a large number of food products are to be inspected on an assembly line. A specific example of this X-ray apparatus is as follows. Food products to be inspected (for example, vegetables such as green peppers, food items such as manufactured bread, or blocks of meat) are placed on a conveyor belt that, for example, advances 60 m per minute. An X-ray generator is set above the belt. In addition, a vertically long X-ray detector is set on the underside of the belt on which the food products are placed, or in other words, in the center of a circulating belt. The X-ray detector has a detection surface that covers the overall width of the line. The detector outputs frame data at a fixed rate. The pieces of frame data are mutually added, for example, synchronously with the movement speed of the conveyor belt.
At present, a detector in which a scintillator and a photoelectric conversion element are combined is often used. A reason for this is to enhance X-ray detection sensitivity to X-ray energy in the range of about 20 keV to 150 keV. A scintillator such as cesium iodine (CsI) or gadolinium oxysulfide (GOS) is typically used as the scintillator. Therefore, the scintillator has a relatively low response speed, has decay characteristics, and has a relatively narrow dynamic range. Consequently, restrictions are applied to apparatus operation on the user side. For example, the object to be inspected is restricted to food products that are thin in thickness and have relatively low X-ray absorption, and the amount of food products fed onto the line is suppressed.
A food product inspection apparatus (foreign matter detection apparatus) that uses a dual-energy detector to even slightly reduce such restrictions is also known. In the food product inspection apparatus, a detector that absorbs low-energy X-rays and a detector that absorbs high-energy X-rays are arranged in an overlapping manner. In the case of this apparatus, a scheme is implemented in which two types of images are separately reconfigured based on the respective frame data outputted from the two detectors. Foreign matter is then visualized through calculation of the difference between the two images. However, resolution is insufficient even in this apparatus. To meet needs, such as the need to check for even small pieces of foreign matter measuring about 0.3 mm, for example, restrictions, such as reducing speed, restricting objects to be inspected to thin objects, and arranging the objects to be inspected in a more dispersed manner, are applied. Stable inspection of the differences in X-ray absorption regarding such small pieces of foreign matter is difficult.
Realistically, when an image of a single tomographic plane (or cross-section) in a height direction on the belt is viewed, detection of foreign matter that is present on the tomographic plane or in a position near the tomographic plane is possible. However, detection of foreign matter that is present away from the tomographic plane or in a three-dimensional manner is difficult.
In consideration of such perspectives, a method and an apparatus for generating images of multiple tomographic planes, described in JP-A-2012-509735, which is described in PTL 1, are also known. The invention described in this publication gives an example in which a photon-counting X-ray detector is combined with a tomosynthesis technique. Images of a plurality of slice planes of a subject are obtained for use in mammography, based on frame data of a desired X-ray energy bin. This tomosynthesis technique is also referred to as a laminography technique in the field of non-destructive inspection.
A similar method for imaging multiple tomographic planes is also described in JP-A-2005-13736, which is described in PTL 2.